An optical modulation device using so-called a Mach-Zehnder type modulator is focused as an optical modulation device operating in a wide wavelength range and in high-speed which is strongly expected as a light source of a WDM communication system capable of performing a high-capacity optical communication. At present, for example, an optical transmitter including a Mach-Zehnder interferometer type optical modulator (hereinafter, referred to just as an LN-MZ type optical modulator) using an electro-optical effect in a nonlinear optical crystal lithium niobate (LiNbO3, LN) and a semiconductor laser is used in a long-distance and high-capacity optical communication system. Besides, in recent years, development of a semiconductor Mach-Zehnder (MZ) type optical modulator (hereinafter, referred to just as a semiconductor MZ type optical modulator) and a semiconductor MZ type optical modulator-integrated semiconductor laser has been in progress.
An example of the semiconductor MZ type optical modulator is illustrated in FIG. 19.
This semiconductor MZ type modulator is made up by including a first optical coupler 101 formed by a semiconductor waveguide, arms 102a, 102b, a second optical coupler 104 and modulating electrodes 103a, 103b. 
The first optical coupler 101 includes an input port 101a, and is an input coupler splitting incident light into two pieces. The arms 102a, 102b are waveguides where two branched lights are propagated. The modulating electrodes 103a, 103b are formed on the waveguides of the arms 102a, 102b, and are electrodes to apply modulation signals to each of the arms 102a, 102b. The second optical coupler 104 includes an output port 104a, and is an output coupler multiplexing (coupling) the lights propagating through the arms 102a, 102b. 
One end of each of the two arms 102a, 102b is connected to the first optical coupler 101 and the other end thereof is connected to the second optical coupler 104. The light incident from the input port 101a to the first optical coupler 101 is branched by the arms 102a, 102b, multiplexed by the second optical coupler 104 after each passing through the arms 102a, 102b, and output from the output port 104a. On/off states of output light are switched depending on an interference state of the light at the multiplexer. When a phase difference of the lights passing through the arms 102a, 102b at the time when they are coupled again at the output port 104a of the second optical coupler 104 is “0” (zero) (or 2Nπ, where “N” is an integer), it becomes in the constructive interference state, and therefore, it becomes the ON state in which the light is output from the output port 104a. On the other hand, when the phase difference of the lights passing through the arms 102a, 102b at the time when they are coupled again at the output port 104a of the second optical coupler 104 is π (or (2N+1)π), it becomes in the destructive interference state, and therefore, it becomes the OFF state in which the light is not output from the output port 104a. 
For example, there is a method adjusting a phase by using a refractive index change generated when an electric field is applied to the arms 102a, 102b and so on as an adjustment method of the phase of the light passing through the arms 102a, 102b. Accordingly, when modulating voltage signals are applied to the arms 102a, 102b by the modulating electrodes 103a, 103b, changes of the phases occur at the arms 102a, 102b in accordance with the modulating voltage signals, and as a result, intensity of the output light is modulated. At first, it is necessary for the modulation signals applied to the arms 102a, 102b to have an amplitude large enough to change the phase difference for π to perform a fine optical modulation. In addition, it is necessary to control the phase difference of the lights passing through the arms 102a, 102b such that the phase difference at an OFF level of the modulating voltage signal is π, and the phase difference at an ON level is “0” (zero). The phase difference of the lights passing through the arms 102a, 102b is different by each modulator element depending on manufacturing errors and so on, and therefore, it is necessary to adjust the phases by each modulator element.
There is a method in which an electrode for phase control is formed at least at one arm in addition to the modulating electrodes 103a, 103b, the refractive index of at least either one of the arms is changed by adjusting the voltage applied to the phase control electrode to change the phase as a method controlling the phase difference between the arms 102a, 102b. There is a method to make a difference in DC biases applied to the modulating electrodes 103a, 103b as another method controlling the phase difference between the arms 102a, 102b. Generally, in the semiconductor MZ type modulator, a modulating operation is performed by applying the DC bias at approximately several V, and it is possible to adjust the phase difference between the arms 102a, 102b by making the DC biases different between the arms 102a, 102b. 
On the other hand, a phenomenon so-called as a wavelength chirp in which a wavelength of the output light changes with a modulation occurs in the MZ type optical modulator. This wavelength chirp becomes a factor deteriorating an optical modulation waveform after an optical fiber transmission. Accordingly, it is necessary to precisely control the wavelength chirp to obtain enough modulation characteristics in the MZ type optical modulator. There are a technology of a zero chirp operation in which an amount of the wavelength chirp is reduced as much as possible, and a technology of a negative chirp operation in which a wavelength chirp improving an optical waveform after transmission is intentionally added as a controlling technology of the wavelength chirp. These chirp controlling technologies are enabled by adjusting a rate of amplitudes of phase changes of the lights generated at the arms 102a, 102b when the modulating voltage signals are applied. In the zero chirp operation, it is desirable that the rate of the amplitudes of the phase changes of the lights generated at the arms 102a, 102b when the modulation signals at high-frequency voltages are applied is fixed to be 1:1. In the negative chirp operation, it is desirable that the rate of the amplitudes of the phase changes of the lights generated at one arm and the other arm is fixed to be, for example, approximately 0.85:0.15.
The above-stated rates are enabled by, for example, adjusting the rate of the amplitudes of the voltage signals applied to the arms 102a, 102b. The modulation signals of which amplitudes are the same and directions are in reverse are applied to the arms 102a, 102b to make the amounts of the phase changes generated at the arms 102a, 102b the same and make the directions in reverse to enable the zero chirp operation. For example, the modulation signals of which amplitudes are different between the arms 102a, 102b and directions are in reverse are each applied to make the one phase change amounts of the arms 102a, larger than 102b, to enable the negative chirp operation.
As stated above, in the MZ type optical modulation device, it is required to appropriately perform both the control of the phase difference between the arms 102a, 102b and the control of the rate of the amplitudes of the phase changes between the arms to obtain the fine modulation characteristics.